The invention relates to an array antenna using microstrip antenna used for two frequencies and inhibitive blocking.
A microstrip antenna using an unbalanced planar circuit generally has the advantage of small size light weight and low loss.
FIG. 12 is a perspective view of the conventional microstrip antenna described in the book, I.J. Bahl, P. Bhartia, "Microstrip antennas" second chapter, p. 31-84, 1980, ARTECH HOUSE, INC. FIG. 12(a) is a perspective view of the conventional microstrip antenna as viewed from the top face. FIG. 12(b) is a perspective view of the conventional microstrip antenna as viewed from the bottom face. In the figure, 1a is a dielectric substrate. 2a is an earth conductor formed on one side of the dielectric substrate 1a. 3 are rectangular radiating conductors having edges L and W formed on another side of the dielectric substrate 1a. 4a are power supplying through holes for supplying microwave energy to the rectangular radiating conductors 3. 5a are clearances for causing the power supplying through holes 4a to cut off the direct current from the earth conductor 2a. 11 are open edges of the radiating conductors which radiate the high frequency band microwave therefrom. 6 is a polarization direction of the main polarized wave radiated from the array antenna.
The operation of the conventional array antenna is explained using FIG. 12(a) and FIG. 12(b). The microwave energy supplied to the plurality of rectangular radiating conductors 3 through the plurality of power supplying through-hole 4a, have current components being parallel to the polarized direction 6 and magnetic current components being orthogonal to the polarized direction 6. An electromagnetic wave is radiated from the rectangular radiating conductors 3 to the space by the current sources and the magnetic current sources which are formed by the current components and the magnetic current components, respectively. The electric field direction of the radiated electromagnetic wave is the same as the polarized direction 6.
The resonance frequency f0 of the fundamental mode of the microstrip antenna is mainly determined by the edge length L of the rectangular radial conductors 3 and the relative dielectric constant .epsilon.r of the dielectric substrate 1a. The frequency band width is also determined by the relative dielectric constant .epsilon.r and the thickness h of the dielectric substrate 1a. The frequency band width is wider if the relative dielectric constant .epsilon.r is smaller and the thickness h is larger. But the selection range of the thickness h is limited in order to suppress the higher mode excitation. The frequency band width of the practical microstrip antenna is about several percents as shown in FIG. 13. FIG. 13 shows the relation between resonance frequency and reflection characteristics of the microstrip antenna used as the conventional array antenna.
An impedance at the power supply points of the power supplying through holes 4a form where the microwave supplied to the microstrip antennas becomes high when the power supplying through-holes 4a are adjacent at the position of the open border edges so that the distance X equals 0. The impedance at the power supply points becomes lower when the power supplying through-holes 4a reach a center of the radiating conductors 3. Therefore, the impedance at the power supply points can be matched with an impedance of a feeding circuit by selecting the distance X.
The dimension Y of the microstrip antenna is selected such as Y=W/2 in order to avoid the generation of the cross polarized wave component.
Since the conventional array antenna is constructed as described above, there are some problems that an array antenna can be used only in a single frequency band when used for a radar antenna, and a plurality of targets cannot be processed at the same time in case where there are more than two targets within the beam search range of the radar.
It is a primary object of the present invention to provide an array antenna which can be used in two frequency bands.
It is another object of the present invention to provide an array antenna which radiates an electromagnetic wave from high frequency band microstrip antenna through the comb-shaped gap of the low frequency band microstrip antennas without receiving the influence of blocking by the comb-shaped low frequency band microstrip antenna.
It is a further object of the present invention to provide an array antenna which improves the angular resolution by diminishing the beam width of the antenna radiation pattern, by changing the operating frequency from lower frequency to higher frequency, when the array antenna is used as a radar antenna.